Embodiments of the invention relate generally to an energy storage device used in an engine starting application and, more particularly, to a supercapacitor array and method of charging thereof in a controlled manner to maximize energy extraction and array lifetime.
Motor vehicles typically utilize a starter motor for starting the vehicle's engine. The starter motor is supplied with electrical energy from a battery, such as a lead-acid storage battery. The battery is typically charged by an alternator while the motor vehicle is running. When the battery is discharged to a level below that of a threshold start voltage, the battery, by itself, cannot supply an adequate current to the starter motor at the time of starting the engine.
With specific regard to heavy trucking applications, it has been previously recognized that double insulated capacitors—i.e., supercapacitors—are beneficial to supplement and indeed replace lead-acid batteries in starting the engine. More specifically, supercapacitors provide a very low impedance high power source that enables a starter motor to successfully crank and start an engine, even under adverse conditions such as extreme cold that might prevent a battery from accomplishing that task. In typical applications, the voltage/potential of the cells of a supercapacitor array (as for engine starting) are held at less than their max rated voltage when the cells are above 0° C. via operation of an associated charger. The cells are held at a lower (i.e., less than their max rated) voltage at such temperatures because it is recognized that excessively charging the cells to a higher voltage may degrade the cells—with FIG. 1 illustrating a reduction in the operating lifetime of cells in a supercapacitor array via curves 2, 4, 6 that show an estimated mean life for a cell when charged to 2.4 V, 2.5 V, and 2.7 V, respectively, at various temperatures. Upon the temperature of the cells dropping below a lower threshold of perhaps 0° C., the charger then automatically increases the voltage/potential of the cells of the supercapacitor array (e.g., to their max rated voltage), such that the energy storage in the supercapacitor array is maximized when it is most needed—during engine start-up at cold temperatures when there is increased viscosity of cold engine oils. In charging the supercapacitor array to its max rated voltage, and in order to not excessively discharge the system battery, a general strategy is for the charger to bring the supercapacitor array up to the max rated voltage and then shut the charger down, reducing its quiescent draw from the battery to something negligible.
While the above described technique for charging the supercapacitor array—with the voltage/potential of the cells of the supercapacitor array being held at a useful level but less than their max rated voltage when the cells are above 0° C. and being increased (e.g., to their max rated voltage) when the temperature of the cells drops below 0° C.—is effective with regard to providing a needed voltage boost during cold weather hard starting of the engine, it is recognized that this technique may not be ideal for maximizing energy extraction from the supercapacitor array. First, because during warm conditions the technique holds the supercapacitor array at a relatively constant voltage indefinitely, that voltage and corresponding stored charge must be reduced to support long life of the cells—providing continuous higher voltage during warm conditions is not an option. Second, as the charger is shut down during cold temperatures upon bringing the supercapacitor array up to its max rated voltage, the self discharge (leakage) currents of the cells and other parasitic loads over the following hours after charging may slowly discharge the supercapacitor array until it hits a lower threshold, at which point the charger turns back on and refreshes the lost charge and again cycles off. Accordingly, if a given design implements 1 V or even just 0.5 V of charge hysteresis, a user cannot be assured when attempting to start a cold truck whether the expected max rated voltage boost is available for the starter or whether instead some lower voltage down to the refresh threshold is only available.
While the technique for charging a supercapacitor array is discussed above with respect to engine starting in a heavy trucking application, it is recognized that other non-mobile industrial applications and machines may have a similar battery-supercapacitor-starter topology. Thus, machines such as large gas or diesel powered generators or pumps may include supercapacitors to ensure engine starting in environments that are difficult for traditional batteries. As set forth above, existing techniques for selectively holding and increasing the voltage/potential of the cells of the supercapacitor array are effective with regard to selectively providing a needed voltage boost during some occurrences of hard starting of the engine, but are not ideal for maximizing energy extraction from the supercapacitor array.
Therefore, it is desirable to provide a supercapacitor array and technique of charging thereof that provides a max rated voltage to be drawn from a supercapacitor array on-demand in heavy vehicle and non-mobile industrial applications. It is further desirable for such an on-demand max rated voltage to be available from the supercapacitor array in such a fashion so as to minimize the impact on the operational lifetime of the supercapacitor array.